FERGUSON, Mo. — When Sam Andrews awoke on Tuesday morning, he found his wife watching a television interview with a woman whose bakery had been vandalized during the violent unrest here on Monday.

“She said, ‘You’ve got to go help her,’ ” Mr. Andrews said in an interview on Saturday morning.

And so Mr. Andrews, a former Defense Department contractor who is now a weapons engineer in the St. Louis area, set to work. Under the auspices of a national group called the Oath Keepers, Mr. Andrews accelerated plans to recruit and organize private security details for businesses in Ferguson, which are receiving the services for free. The volunteers, who are sometimes described as a citizen militia — but do not call themselves that — have taken up armed positions on rooftops here on recent nights.

“It’s really a broad group of citizens, and I’m sure their motivations are all different,” said Mr. Andrews, who is in his 50s. “In many of them, there’s probably a sense of patriotism. But I think in most of them, there’s probably something that they probably don’t even recognize: that we have a moral obligation to protect the weakest among us. When we see these violent people, these arsonists and anarchists, attacking, it just pokes at you in a deep place.”

Mr. Andrews declined to say how many people were assisting in the effort, saying only that the number was “more than five, less than 500.” He estimated that men make up about 80 percent of the volunteers. About 80 percent are white, and 10 percent are black.

But on Saturday, with the county police said to be threatening the Oath Keepers with arrest, the volunteers decided to abandon their posts and instead protest against the authorities. During the evening, Mr. Andrews and some of his colleagues appeared on South Florissant Road, conducting a protest of their own. They ate pizza and stood beneath a handmade sign critical of Chief Jon Belmar of the St. Louis County police.

Just after 8:30 p.m., those protesting Mr. Brown’s death crossed paths with the Oath Keepers. Mr. Andrews shook hands with the demonstrators, many of whom have also been critical of Chief Belmar.

That some business owners accepted aid from a group regarded by some as an antigovernment militia is a testament to the rawness of emotions here following a riot on Monday night, after a grand jury declined to indict a white police officer, Darren Wilson, who shot to death Michael Brown, an unarmed black teenager, on Aug. 9.

Since then, the riot police have tussled on the streets with protesters. Convoys of National Guard Humvees patrol the streets, and law enforcement officials have confiscated guns, firecrackers and homemade firebombs from arrested protesters. The Oath Keepers, equipped with militia-style gear and often clad in camouflage, have joined the volatile mix.

Their presence is a symbol of the continuing criticism of Gov. Jay Nixon’s handling of security before and after the grand jury’s decision became public. In the days before the announcement, Mr. Nixon declared a state of emergency and sent the Guard to Ferguson. Yet, in the initial hours following word of the grand jury’s decision, the Guard played only a limited role. Troops protected a police command post and other facilities, but they were not posted along the main commercial corridors where property destruction was rampant.

“The governor said himself, I think on TV, that the businesses will be protected by the National Guard during this situation,” said Reggie Jones, the mayor of Dellwood, a neighboring city where many businesses were damaged in Monday’s unrest. “Unfortunately, that did not happen until Tuesday. But we were expecting it to happen on Monday.”

But for people like Mr. Andrews, the governor’s vow was of little solace. So while the New Chinese Gourmet restaurant at the end of a block of South Florissant Road appears to have little in the way of defense beyond the painted wooden boards that cover its windows, armed men and women on recent nights have roamed the rooftop it shares with a dental practice and a law office.

“When they’re here, there’s definitely a weight lifted off of our shoulders,” said Davis Vo, whose family owns New Chinese Gourmet. “I’d be lying if I said otherwise.”

On its website, Oath Keepers released a recruiting message to “all skilled veterans and patriots” and asked them to “grab your gear and start rolling toward Ferguson.” The post listed nine types of people the group was seeking, including paramedics, police officers, “private drone operators” and videographers who could “film any encounters with looters.”

Mr. Andrews said he researched the qualifications of each volunteer, as well as whether any might have racially based motivations to participate.

“I don’t want any racists in my group,” he said. “I don’t want any people who want to visit violence on any group. I only want professionals with real credentials that can be verified and have experience in dealing with violence.”

But the St. Louis County police, Mr. Andrews said, and other law enforcement officials have expressed misgivings.

“When we hear information that someone, or a group, is providing security without a license, our department has to investigate the issue,” a police spokesman, Shawn McGuire, said in an email on Saturday.

Mr. Andrews said that the warning on Friday was tantamount to a temporary shutdown order, and he said he did not expect his volunteers to defy it.

A version of this article appears in print on , on Page A33 of the New York edition with the headline: On Rooftops of Ferguson, Volunteers — With Guns. Order Reprints | Today’s Paper | Subscribe